Method of manufacturing lead-sheathed cables



United States Patent METHOD OF MANUFACTURING LEAD-SHEATHED CABLES John F. Eckel, Scotia, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application September 9, 1952,

Serial No. 308,710

4 Claims. (Cl. 148-125) The present invention relates to a method of mauufacturing lead sheathed cables. More particularly, it is concerned with a method of improving the physical properties of the lead sheath comprising such cables.

In the manufacture of lead-sheathed cables, the sheath is applied by extruding a layer of lead about the insulated cable as the cable is passed through a suitable die block. In order to insure a satisfactory extrusion rate, the die block and the body of lead being extruded onto the cable are ordinarily held at a temperature somewhat above room temperature and ordinarily in the neighborhood of about 200 C. The physical properties of the extruded sheath are largely determined by the extrusion temperatures and the rate of cooling of the sheath after emergence from the die block. Heretofore, with the lead base alloys commonly employed as cable sheathing alloys, the usual heat treatment has consisted of water quenching the lead alloy sheath from the elevated temperature at which it emerges from the die block for the purpose of obtaining a solution-hardened alloy which was considered to possess the optimum physical properties required in a cable sheathing alloy. Because the usual lead base alloys are quite sensitive to variations in the die block temperatures from which the alloys are quenched and because these temperatures, for various reasons, are not uniform throughout long lengths of the cable, various extrusion methods have been proposed for insuring a more uniform control of the sheathed temperatures at the time the quenching operation is performed. One such method is described in Patent 2,504,600-Snyder. the sheathed cable immediately after emergence from the die block is passed through a hot liquid, the temperature of which is closely controlled substantially at the desired quenching temperature in order to insure a uniform quenching temperature regardless of die block temperatures. While the sheath is at substantially the desired temperature, it is quickly quenched by means of a cold quenching liquid. The primary purpose of this process is to obtain a uniform hardening of the sheath throughout the entire cable length. The process is based on the supposition that the best properties are obtained in a heattreatable lead alloy by controlling the amount of solution hardening so that it is essential to the process that the quenching be done from a constant temperature which is best adapted to obtain the optimum solution hardening.

The solution hardening heat treatment has been most useful in connection with the heat-treatable lead alloys containing small amounts of arsenic and bismuth or arsenic, bismuth and tin. The solution-hardening treatment has also been successfully applied to tellurium-containing lead alloys of a heat-hardenable type containing as essential alloying ingredients small amounts of tellurium and arsenic or tellurium, arsenic and tin. Alloys of this type are more specifically described and claimed in Patent 2,588,095-Eckel.

The tellurium-arsenic and particularly the tellurium- In accordance with that process,

, erably from about 20 to 60 seconds.

2,709,144 Patented May 24, 1955 arsenic-tin lead base alloys are specifically characterized by good creep resistance at high operating temperatures and good bending life under high strain conditions so that the cable sheaths composed of these alloys are characterized by an improved service life particularly at the more elevated operating temperatures.

The heat-treatable tellurium-containing lead alloys found useful for cable sheath applications are those containing from 0.05 to 0.2% tellurium, 0.15 to 0.3% arsenic. For optimum properties the alloys should also contain from 0.05 to 0.2% tin, balance substantially lead.

The present invention is particularly concerned with an improved heat treatment designed to further enhance the physical properties of cable sheaths composed of the tellurium-containing lead base alloys. A primary object of the present invention is to provide a heat treatment for lead cable sheaths composed of the tellurium-containing lead base alloys adapted to render such alloys more resistant to inter-granular fracture resulting either from a long time application of stress or from repeated bending. A further object of the invention is to provide a heat treatment for such cable sheath alloys designed to improve the ductility characteristics of the sheathing alloy. Another object of the invention is to provide an improved heat treating process for such cable sheath alloys which is both economical and easily adaptable to the usual cable sheathing process.

The heat treatment which has been found to develop these improved properties in lead base cable sheath alloys containing both tellurium and arsenic briefly comprises quenching the lead sheath to substantially room temperature by a suitable liquid coolant, such as oil or water, immediately after the hot extrusion thereof onto the cable followed by a reheating of the quenched sheath to a temperature somewhat below the usual die block temperatures for a period of from about 5 to 60 seconds and finally air cooling the reheated sheath to a temperature not exceeding about C.

Whereas the previous heat treatment supplied to heattreatable lead cable sheath alloys were of the solutionhardening type, the heat treatment of the present invention is primarily designed to increase and control the amount of secondary phase precipitated within the grain boundaries of the alloy in order to render the grain boundaries more strongly resistant to inter-granular frac ture under high stress or excessive bending conditions.

In the practice of the present invention, it is desirable that the amount of solution hardening be minimized as much as possible. To accomplish this and at the same time adapt the process to the usual cable sheathing procedure, the sheath is quenched immediately after emergence from the die block using any workable die block temperatures preferably within a range of from about 210 C. to 220 C. Lower die block temperatures may be employed but they materially reduce the extrusion rate. Slightly higher die block temperatures are likewise satisfactory, but care must be observed to prevent overheating and the resultant feathered surface on the sheath. As the cable emerges from the water or oil quenching medium, it is given a precipitation or over aging treatment by being immediately reheated to a temperature between C. and 210 C., preferably 200 C. for a short time interval of from 5 to 60 seconds, pref- Best results are obtained of this time interval at about 30 seconds at a temperature of 200 C. This reheating can be accomplished by means of a liquid bath, a liquid spray or a liquid flooding of the moving cable. The reheated cable is then air-cooled to a temperature not exceeding 140 C. after which it is ready to be wound on the usual take up reel.

The improvements obtained in accordance with the present invention can best be illustrated by a comparison of the properties of the heat treated alloy with the properties of the same alloys which have been subjected to a solution-hardening treatment as described in the abovementioned Eckel Patent 2,588,095.

The precipitation or over-aging heat treatment of the present invention has no significant effect on either the bending life or creep properties of the tellurium-containing alloys as obtained by the usual solution-hardening treatment. In other words, the excellent creep and bending life characteristics of the solution-hardened telluriumcontaining alloys are not materially affected by the precipitation or over-aging heat treatment. On the other hand, the ductility characteristics of the alloy, particularly its ductility'in rupture tests and tensile tests is materially improved. As a result, in these tests there is obtained a ductile knife-edge fracture rather than the brittle intergranular fracture characteristic of the solution-hardened alloys.

As an example of the results obtainable by the present invention, an alloy composed, by weight, of 0.12% tellurium, 0.20% arsenic, 0.13% tin, balance the commercially available lead known in the trade as common lead was extruded as a cable sheath at an extrusion die block temperature averaging 219 C. and quenched to room temperature immediately after emergence from the die block. Tests were made on samples of the sheath in the quenched condition. Tests were also made on samples of the same sheath after it had been given the precipitation or over-aging treatment. No significant differences were noted in the bending and creep properties. However, the ductility of the over-aged material was much higher.

The improvement in properties obtainable through the present heat treatment is believed to result from two changes that occur in the microstructure of the alloy. First, a more complete film of grain boundary precipitate is formed and second, precipitation proceeds within the grains themselves as the result of the over-aging heat treatment, thus materially increasing the ductility of the alloy in the rupture and tensile tests.

The rate at which these changes occur is considerably enhanced by the quench given after emergence from the die block since the degree of supersaturation is more substantial at the time precipitation begins. Furthermore, by starting precipitation on a heating cycle following the quench, better dispersion and more uniform distribution of the second phase along grain boundaries will result. In addition, the eflfect of press stop marks on the tensile strength of the sheath is almost completely eliminated resulting in more uniform properties throughout the entire length of the cable.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the manufacture of a lead-sheathed cable in which the lead sheath is an alloy consisting essentially, by weight, of 0.05 to 0.4% tellurium, 0.05 to 0.5% arsenic, balance substantially all lead, the method of improving the physical properties of the extruded lead sheath which comprises quenching the sheath to substantially room temperature immediately after hot extrusion thereof onto the cable, reheating the quenched sheath to a temperature of 190 C. to 210 C. for from 5 to seconds and air cooling the reheated sheath to a temperature not exceeding about C.

2. In the manufacture of a lead-sheathed cable in which the lead sheath is an alloy consisting essentially, by weight, of 0.05 to 0.4% tellurium, 0.05 to 0.5% arsenic, 0.05 to 0.2% tin, balance substantially all lead, the method of improving the physical properties of the extruded lead sheath which comprises quenching the sheath to substantially room temperature immediately after hot extrusion thereof onto the cable, reheating the quenched sheath to a temperature of C. to 210 C. for from 20 to 60 seconds and air cooling the reheated sheath to a temperature not exceeding about 140 C.

3. In the manufacture of a lead-sheathed cable in which the lead sheath is an alloy consisting essentially, by weight, of 0.05 to 0.4% tellurium, 0.05 to 0.5% arsenic, 0.05 to 0.2% tin, balance substantially all lead, the method of improving the physical properties of the extruded lead sheath which comprises oil-quenching the sheath to substantially room temperature immediately after hot extrusion thereof onto the cable, reheating the quenched sheath to a temperature of about 200 C. for about 30 seconds and air-cooling the reheated sheath to a temperature not exceeding about 140 C.

4. The method of manufacturing a sheathed cable comprising the steps of extruding a sheath of an alloy consisting essentially, by weight, or" 0.05 to 0.4% tellurium, 0.05 to 0.5 arsenic, balance substantially all lead, upon a cable passing through suitable extrusion dies, said dies and said alloy being maintained at a temperature of substantially 200 C. during extrusion, immediately quenching the sheathed cable to substantially room temperature by passing it into a quenching media maintained at the necessary'temperature, reheating the quenched sheath as it emerges from the quenching media to a temperature of 190 to 210 C. for from 5 to 60 seconds, and air cooling the reheated sheath to a temperature not in excess of 140 C. before it is reeled.

References Cited in the file of this patent UNITED STATES PATENTS 1,926,545 Koch Sept. 12, 1933 2,148,741 Gonser Feb. 28, 1 939 2,375,755 Bassett, Ir. May 15, 1945 2,588,095 Eckel Mar. 4, 1952 

1. IN THE MANUFACTURE OF A LEAD-SHEATHED CABLE IN WHICH THE LEAD SHEATH IS AN ALLOY CONSISTING ESSENTIALLY, BY WEIGHT, OF 0.05 TO 0.4% TELLURIUM, 0.05 TO 0.5% ARSENIC, BALANCE SUBSTANTIALLY ALL LEAD, THE METHOD OF IMPROVING THE PHYSICAL PROPERTIES OF THE EXTRUDED LEAD SHEATH WHICH COMPRISES QUENCHING THE SHEATH TO SUBSTANTIALLY ROOM TEMPERATURE IMMEDIATELY AFTER HOT EXTRUSION THEREO INTO THE CABLE, REHEATING THE QUENCHED SHEATH TO A TEMPERATURE OF 190* C. TO 210* C. FOR FROM 5 TO 60 SECONDS AND AIR COOLING THE REHEATED SHEATH TO A TEMPERATURE NOT EXCEEDING ABOUT 140* C. 